In VLBI, data from two or more telescopes are combined to form a single, virtual telescope that spans the geographic distance between them. The most recent of these experiments with ALMA and other telescopes formed an Earth-sized telescope with extraordinarily fine resolution.

These experiments are an essential step in including ALMA in the Event Horizon Telescope (EHT), a global network of millimetre-wavelength telescopes that will have the power to study the supermassive black hole at the centre of the Milky Way in unprecedented detail.

Before ALMA could participate in VLBI observations, it first had to be transformed into a different kind of instrument known as a phased array [1]. This new version of ALMA allows its 66 antennas to function as a single radio dish 85 metres in diameter, which then becomes one element in a much larger VLBI telescope.

The first test of ALMA’s VLBI capabilities occurred on 13 January 2015, when ALMA successfully linked with the Atacama Pathfinder Experiment Telescope (APEX), which is about two kilometres from the centre of the ALMA array.

On 30 March 2015, ALMA reached out much further by linking with the Institut de Radioastronomie Millimetrique’s (IRAM) 30-metre radio telescope in the Sierra Nevada of southern Spain. Together they simultaneously observed the bright quasar 3C 273 [2]. Data from this observation were combined into a single observation with a resolution of 34 microarcseconds. This is equivalent to a separation of less than ten centimetres on the Moon, seen from Earth, or less than half a light-year as seen at the quasar’s distance of 2.5 billion light-years.

The most recent VLBI observing run was performed on 1–3 August 2015 with six of the National Radio Astronomy Observatory’s (NRAO) Very Long Baseline Array (VLBA) antennas [3]. This combined instrument formed a virtual Earth-sized telescope and observed the quasar 3C 454.3, which is one of the brightest radio beacons on the sky, despite lying at a distance of 7.8 billion light-years. These data were first processed at NRAO and MIT-Haystack in the United States and further post-processing analysis is being performed at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany.

The new observations are a further step towards global interferometric observations with ALMA in the framework of the Global mm-VLBI Array and the Event Horizon Telescope, with ALMA as the largest and the most sensitive element. The addition of ALMA to millimetre VLBI will boost the imaging sensitivity and capabilities of the existing VLBI arrays by an order of magnitude.